Is it necessary to impregnate a sofa?

Impregnation is not always necessary. If you have a 100% wool fabric, then wool fat (lanolin) usually provides sufficient protection. For light colours it is often sensible. 

Sometimes you also see combinations of, for example, 70% wool and other fibres. If you have 30% viscose in the fabric, then impregnation certainly has added value. 

Even if you have thoroughly cleaned a somewhat older fabric and removed the lanolin, impregnation is useful to protect the fabric again.